The present invention relates to a die button associated with a method for the punching in and attachment of a fastener element to a planar workpiece, in particular to a sheet metal part.
During the punching in of the fastener element into a workpiece the problem frequently arises that swarf is generated by the punching process and disadvantageously affects the reliability of the attachment of the element. The swarf is often jammed between a rivet section, which is beaded over during or after the actual punching in of the fastener element for its attachment to the workpiece, and the workpiece. In some cases, the swarf also remains lying in a die button which is used for the punching in and the beading over of the rivet section. The swarf is then lifted with the punched in fastener element secured to the workpiece out of the die button and subsequently migrates as a foreign body through the tooling and can lead to ugly impressions in the finished component assembly in the further course of the manufacture of a component.
It is thus an object of the present invention to provide a method of the initially named kind in which the creation of swarf is avoided.
The solution of this object takes place by a method having the features of claim 1.
In accordance with the invention a movably arranged die plunger of the die button is pressed with a predetermined pressing force against the workpiece during the punching in of the fastener element into the workpiece. The pressing force is adapted during this in such a way that the die plunger first moves when a slug has been cut out of the workpiece by a piercing section of the fastener element.
It has namely been recognized by the inventors that, in a customary punching in process, the piercing and rivet section of the fastener element first creates a depression in the workpiece which projects into the die button. In this connection the workpiece is strongly stretched at the side opposite to the piercing and rivet section. A crack arises at the position of the smallest banding radius of the elongated region and does not necessarily coincide with a cut edge of the slug to be cut out by the die button and the piercing and riveting section. This signifies that during the punching in process a bur, in particular a ring-shaped burr, arises which causes the initially named problems.
In accordance with the invention this problem is prevented in that the workpiece is supported during the punching in process in which the slug is cut out from the workpiece. This is achieved by the die plunger which is pressed against the workpiece from a side of the workpiece opposite to the fastener element which is to be punched in. In this way, the creation of the depression which initially occurs in the customary process is avoided. Only when the cutting process associated with the punching in is finished, i.e. as soon as the slug has been substantially completely separated from the workpiece is a movement of the die plunger permitted. Basically, provision can be made that a movement of the die plunger is first permitted after the completion of the cutting process. It is however also possible to provide a movement of the die plunger after a delay time, i.e. when a certain portion of the piercing section of the fastener element has already passed through the workpiece. The direction of movement of the die plunger takes place during this in the direction of movement of the fastener element.
It is also basically possible for the die plunger to be actively moved. A passive movement of the die plunger is however preferred. In this connection, one exploits the fact that forces vary which occurring the punching in process. The predetermined pressing force can thus be selected such that a movement of the die plunger automatically first arises in specific states, i.e. specifically first after the slug has been punched out of the workpiece by the fastener element and the forces acting between the die plunger and the fastener element exceed the pressing force.
Further embodiments of the method of the invention are set forth in the description, in the claims and in the accompanying drawings.
In accordance with one embodiment of the method, the die plunger is movable in a direction parallel to a punching in movement of the fastener element. The pressing force, i.e. the force with which the die plunger is biased towards the workpiece is in particular directed antiparallel to the punching in movement of the fastener element. The pressing force can be selected in such a way that the die plunger is first moved when a fastener section of the fastener element is deformed by a shaping surface of the die plunger, i.e. the preset force is so large that the die plunger is first set in motion when, as a result of the shaping of a section of the fastener element, for example a part of the piercing section acting as a rivet section, the forces which occur exceed the pressing force.
In accordance with an embodiment of the method the die plunger has a raised portion which at the start of the punching in process projects out of the plane which is defined by a rim of a recess of the die button. In other words, the die plunger supports the workpiece during the punching in not only in a plane which is defined by a workpiece support surface on which the workpiece lies during the process. The raised portion of the die plunger ensures that a center part of the slug which is to be punched out is be pressed towards the fastener element which is to be punched in. This facilitates the cutting during the penetration of the piercing section into the workpiece. In particular the die plunger includes a cutting edge which at the start of the punching in process lies in a plane which is defined by the rim of the recess of the die button.
In accordance with one embodiment a movement of the die plunger opposite to and/or in the direction of the pressing force is restricted by an abutment in order, on the one hand, to fix a well-defined position of the die plunger prior to the punching in of the fastener element and, on the other hand, to ensure a controlled attachment of the fastener element to the workpiece by fixing a maximum deflection of the die plunger.
The die plunger can have a shaping surface through which the piercing section is at least regionally deformed radially outwardly. For example, the piercing section which also serves as the rivet section is beaded outwardly by the shaping surface. The shaping surface can be conically and/or concavely shaped, at least regionally.
Provision can be made that during the punching in and attachment of the fastener element a depression is generated which at least partly receives a flange of the fastener element which contacts the workpiece in the end state. The depression is in particular at least so deep that the flange does not project in the axial direction out of the depression.
The predetermined pressing force can be selected in dependence of the properties of the workpiece in dependence on the properties of the fastener element and/or in dependence on a pressing force with which the fastener element is pressed by a pressing unit towards the workpiece, in order to ensure that a reliable support of the workpiece is present in the region of the slug which is to be cut out during the cutting process on the punching in of the fastener element.
Thus, a premature creation of a depression which stretches the material of the workpiece and thus leads to the creation of swarf is avoided.
An adjustable biasing device can be associated with the die plunger with which the predetermined pressing force is adjustable.
The invention further relates to a die button for the punching in and attachment of the fastener element to a planar workpiece, in particular to a sheet metal part in accordance with a method in accordance with at least one of the previously described embodiments, with the moveably arranged die plunger being capable of being pressed by a pressing device against the workpiece and wherein a pressing force made available by the biasing device is selected such that the die plunger first moves when a slug has been cut out by the piercing section of the fastener element. The direction of movement of the die plunger takes place during this in the direction of movement of the fastener element.
In accordance with one embodiment the predetermined pressing force generated by the biasing device is adjustable.
The biasing device in particular includes a spring. The biasing device can be designed both as a mechanical spring, such as a coil spring, and also as a pneumatic spring, for example a pneumatically actuated piston in cylinder arrangement or as a hydraulic spring, for example a hydraulically actuated piston in cylinder arrangement, or as a (movable) latch element. Moreover, the die plunger can be moved by an actuator between the two preset positions which represents an equivalent of the biasing device and which can for example be realized in an electrical pneumatic or hydraulic manner.